This invention relates the method and means of dry processing a semiconductor wafer and particularly for those dry processes that provide a gaseous atmosphere against the processed surface of the wafer in which a gas constituent is obtained from a liquid fluid source.
Fabrication of integrated circuits on a semiconductor wafer substrate involves a number of processes including photolithography processes that impose a pattern in photoresist material on the substrate that varies in its resistance to chemical development or dry etching depending upon exposure to a beam of radiation. The beam of radiation may be a light beam, an electron beam (E-beam), X-rays or an ion beam. The development process leaves patterns of the resist material at the surface of the wafer upon which other processes such as deposition, impurity implantation and other chemical processes are carried out.
The photolithographic process usually begins with preparation of the wafer for coating. That preparation may include cleaning, dehydration and priming. Heretofore, many of these pre-coating steps have involved applying a liquid fluid to the wafer surface.
Cleaning requires flooding the wafer with water and scrubbing with a roller brush scrubber or high pressure spraying with water, followed by rinsing the water to insure complete removal of fluids containing contaminates. Residues are removed by high temperature baking in combination with exposure to liquid hexamethyldisilazane (HMDS) and sometimes in combination with vacuum and vapors of HMDS. This is called priming. These and other conventional steps have been used to reduce defect density by removing sub-micron particles and preparing and promoting photoresist adhesion.
Adhesion of the photoresist to the semiconductor wafer surface is compromised by contaminates or water on the surface. Wafers that have been exposed to humidity or that have had direct contact with water during cleaning processes may have a mono layer of water attached to the surface by Vanderwal forces. Failure to remove the water may result in compromising the bond between the photoresist and the semiconductor surface resulting in high defect density. Photoresist has conventionally been accomplished by chemically changing the surface of the wafer by:
1. Removing the mono-layer of water with heat (360.degree. C.). PA1 2. Silylating the semiconductor surface by exposure to organometallic reagents like HMDS. PA1 (a) Flooding the wafer surface with liquid reagent. For example, HMDS is puddled on the surface to prime it. PA1 (b) Expose the wafer surface to vapors of the reagent and nitrogen at atmospheric pressure (760 Torr) and elevated wafer temperature. An example of this technique is described in U.S. Pat. No. 4,556,785, entitled "Semiconductor Wafer Baking Apparatus", issued 12/03/85 to J. Blechschmi, R. D. Coyne, D. Palmer and J. A. Ptatt. PA1 (c) Expose the wafer surface to vapors of the reagent in a sealed container (vapor pressure 1 to 16 Torr) and heat the wafer. For example, the system is loaded in an oven and the liquid reagent is heated to raise its vapor pressure.
Methods of exposing the surface of an uncoated semiconductor wafer to a primary silylating reagent like HMDS include the following:
At this time, the above methods are performed by equipment available from several sources.
The coating process in present use is a wet process and is suitable for use in conjunction with the method and apparatus of the present invention. Coating materials are positive or negative resist. Some of these are developed wet and some are developed by heat or by auto-evaporation of exposed resist. Wet development of both positive and negative resist is the most widely used. Self-development resists have had limited use due to their inability to stand up to dry process demands of advanced manufacturing methods such as reactive ion etching (RIE).
Developing of positive or negative resist is conventionally a wet process and involves the use of spinners for the spray application of liquid reagents formulated to remove the resist selectively and so transfer the radiation image through the resist to the semiconductor wafer surface to be processed (etched). Perfection of these wet develop techniques have for the most part addressed the problem of developing uniformity and do little or nothing to reduce particles produced during the wet developing, wet etching processes.
Etching must be clean reliable and accurate to be suitable for VSLI line widths. That need has motivated development of techniques of processing semiconductor wafers in a vacuum using reactive ion etching (RIE). The method and apparatus of the present invention teaches developing and etching of both positive and negative photoresists in an RIE process after exposure.
Self developing photoresists and the techniques for using them will become more useful when apparatus according to the present invention is available to maintain wafer to wafer consistency. The present invention teaches a method and apparatus for stabilizing the photoresist and making the resist more resistant to damage, particularly when the image is transferred to the semiconductor surface during RIE.
Wet Process Photolithography
As mentioned above, many of the processing steps involve applying a liquid fluid to the wafer surface. For example, the wet process for carrying out photolithography requires flooding, spraying or immersing the wafer in a liquid fluid chemical under controlled conditions calculated to accomplish the process. That process is terminated by removing the liquid from the wafer and is followed by rinsing the wafer with another liquid to ensure complete removal of the first liquid. There must not be any residue of the process liquids left on the wafer and particular handling techniques must be followed to ensure this. An advantage of dry processing where a gas fluid is used rather than a liquid fluid avoids these problems of residue and replaces many steps with one.
Dry Process Photolithography
A dry photolithography process usually begins with treating the surface of a silicon wafer. Then the wafer surface is coated with a photoresist by a wet process, because that is the best known way of adding the photoresist. Next the photoresist is exposed to radiation through a mask pattern and then steps are taken to develop the pattern including removing the unexposed parts of the etchable substrate in which the photoinitiator is not exposed. As mentioned above, perfection of this dry photolithography process has led to a technique of priming the wafer before the photoresist is applied to it. The effect of the priming step is to ensure adherence of the photoresist to the silicon so that it will not separate during the following processing. From such separation, small pieces of exposed (imaged) resist are lost during the development, degrading the process.
Priming Processes
One priming process, in effect, is a silylation process that uses silane vapor. The silane vapor can be obtained from vaporizing an organosilicone compound such as liquid silane sold under the trade name HMDS. One source of such HMDS is marketed under the trademark HMDS PLUS and is a hexamethyldisilazane. Another suitable from of HMDS is glycidoxypropyltrimethoxysilane which has been produced by IBM.
Photoresist Silylation To Resist Etching
Another silylation process, in effect, coats the radiation exposed (imaged) photoresist with organometallic polymers which are particularly resistant to plasma etching. As a consequence, the subsequent dry etching process can then be accomplished by reactive ion etching (RIE), applying a plasma such as oxygen or halocarbon plasma or another RIE gas has little effect on the organometallic silylated resist which forms only where the photoresist has been exposed to radiation, whereas RIE removes the organometallic monomers that remain unbonded in the resist where it has not been exposed to radiation; and, of course, removes the unsilylated resist layer below it.
Such a process for HMDS silylation of the surface of the photoresist substrate for preparing a negative relief image is described in U.S. Pat. No. 4,551,418 which issued Nov. 5, 1985 to Hult et al and is entitled Process for Preparing Negative Relief Images With Cationic Photo Polymerization. That patent describes the process of HMDS silylation as including steps of coating a silicon substrate with an etchable substrate such as a polymeric layer containing at least at its surface a cationic photoinitiator. The photoinitiator is exposed to radiation through a mask (a pattern or radiation) and then contacted with the HMDS at vapor pressure between 6 and 500 Torr. The HMDS is susceptible to cationic polymerization and so forms a polymer on the polymeric layer where the cationic photoinitiator has been exposed to radiation. This polymerized monomer formed from the HMDS ranges in thickness from a few angstrom units to several microns and the polymerized products contained in it include organometallic polymers which are particularly resistant to plasma etching. In this way, a thin layer is formed on the photoresist substrate that is highly resistant to plasma etching where the photoresist surface has been exposed to radiation.
It is an object of the present invention to provide a method and means of performing all of the above mentioned silylation steps to increase the resistance of the radiation exposed photoresist to plasma etching and in addition perform the steps of priming and several steps of baking in a predetermined sequence on a given wafer without removing the wafer from the apparatus and in which all of the processes are dry processes.
Handling Reagents
HMDS and similar organosilicone compounds are highly volatile, flammable liquids. Heretofore, processes like priming have been carried out with HMDS at approximately 6 Torr and the results have been reasonably satisfactory. Priming has not been carried out at pressure greater than 6 Torr even though there is evidence that the priming process would be carried out faster and perhaps more effectively at a higher pressure (higher concentration of HMDS gas). Similarly, silylation with organosilicone compounds such as HMDS in the vapor phase have been carried out at 6 Torr although that process is preferable carried out at at least 200 Torr HMDS vapor pressure which requires heating the HMDS liquid to over 70.degree. C. in order to produce the 200 Torr vapor pressure. At that temperature the HMDS will flash into combustion in the presence of oxygen and so is a hazard.
It is another and further object of the present invention to provide a method and means of producing HMDS gas and vapor at pressures on the order of 200 Torr while avoiding the hazardous conditions mentioned above and encountered in experimental work done in the past.